1. Field of the Invention
The present invention relates to an apparatus for holding plates such as semiconductor wafers having different sizes and thicknesses by a common holding portion.
2. Description of the Related Art
A conventional semiconductor wafer holding apparatus as a typical holding apparatus of this type is shown in FIGS. 1A and 1B. This holding apparatus vertically moves or conveys semiconductor wafers 1. A blade 2 constituting a holding portion has an arcuated upper edge 4 so as to match with a peripheral curve (almost an arc) of the wafer 1, as shown in FIG. 1A. A vertically tapered concave groove 3 is formed to receive and support the edge of the wafer 1 as shown in a sectional view of FIG. 1B along the line 1B--1B of FIG. 1A. Therefore, the edge of the wafer 1 is held in the groove 3. The holding apparatus is used to take up the wafer and hold it in a vertical state or, to hold or move a plurality of wafers, for example twenty five wafers taken out from a wafer carrier or wafer boat, at the same time in a manner that the wafers are kept at regular intervals and in horizontal states.
The groove 3 is set to have a width slightly larger than a maximum width of the wafer 1 in consideration of a tolerance. The holding apparatus described above presents the following various problems.
A. Since the opening width of the groove 3 is considerably larger than a normal wafer thickness, the normal wafer cannot be stably supported at the upper end of the holding portion, resulting in poor positioning precision.
B. In order to precisely match the width of the groove 3 with a wafer thickness, an allowance for a tolerance of groove formation becomes strict, and an exclusive jig for forming a groove is required, resulting in high cost.
C. A process surface of a wafer is brought into contact with a side surface of the groove 3 although a contact area is small.
D. Since there is no element for fixing a wafer, the wafer surface slightly vibrates in a direction parallel to the wafer surface and a direction perpendicular thereto.